The term “terahertz” (THz), originally used by sub-millimeter spectroscopists to describe spectral lines with frequencies in the trillions of hertz, today is applied to broadband pulsed electromagnetic (EM) radiation between the infrared and microwave bands of the EM spectrum, ranging from 0.1 to 10 THz. The photon energies of THz radiation range from 0.4 to 80 mev, which corresponds to the range of fundamental energies associated with changes between molecular energy levels.
Over the past 20 years the technologies in the THz field have developed significantly, both in the generation of THz radiation as well as in its detection. Recent advances in high-speed optoelectronic and femtosecond laser technology facilitate generation and detection of short bursts of terahertz radiation, which has proven to be extremely useful for spectroscopic measurements in the sub-millimeter-wave range. Terahertz imaging combines these coherent spectroscopic measurements with real-time imaging and advanced signal processing and recognition, so that each pixel element of the image contains spectroscopic information about the object. Terahertz radiation is described in greater detail in an article by M. Nuss entitled “Chemistry is Right for T-Ray Imaging,” Circuits & Devices, IEEE (March, 1996).
Typical apparatus and associated imaging methods for free-space electro-optic characterization of propagating terahertz beams are described in U.S. Pat. No. 5,952,818 issued on Sep. 14, 1999 to Zhang et al. and assigned to the assignee of the present invention, Rensselaer Polytechnic Institute. The sensing technique is based on a non-linear coupling between a low-frequency electric field (terahertz pulse) and a laser beam (optical pulse) in an electro-optic crystal, such as a zinc telluride (ZnTe) crystal. Modulating the crystal's birefringence by applying the polarized electric field to the crystal modulates the polarization states of an optical probe beam passing through the crystal. This ellipticity modulation of the optical beam is then polarization-analyzed to provide information on both the amplitude and phase of the applied electric field.
A further improvement in terahertz imaging is disclosed in U.S. Pat. No. 6,414,473 issued on Jul. 2, 2002 to Zhang et al. and also assigned to Rensselaer Polytechnic Institute. The described imaging system in this reference employs a chirped optical beam and dynamic subtraction to rapidly reconstruct an image, thereby providing a system that is suitable for real-time imaging applications. According to this patent, the imaging system generates a free-space electromagnetic radiation pulse positionable to pass through the object to be imaged. One of an electro-optic crystal or a magneto-optic crystal is positioned so that the electromagnetic radiation pulse passes through the crystal after passing through the object.
The system further generates a chirped optical probe signal to impinge the crystal simultaneously with the electromagnetic radiation pulse passing through the crystal so that a temporal waveform of the radiation is encoded onto a wavelength spectrum of the chirped optical probe signal. The chirped optical probe signal modulated by the free-space radiation is then passed to a decoder for decoding a characteristic of the free-space electromagnetic radiation using the chirped optical probe signal with the temporal waveform of the radiation encoded on the signal. The system further determines a characteristic of the object using the characterization of the free-space electromagnetic radiation pulse after passing through the object.
Both of the patents summarized above are incorporated, in their entirety, in this document by reference.
Tomography refers to the cross-sectional imaging of an object from measuring either the transmitted or reflected illumination. Three-dimensional THz tomography coupled with spectroscopic analysis has many potential applications such as mail package examination, security screening, and nondestructive inspection. In 1997, Mittleman et al. demonstrated THz tomography in which the 3D image of a floppy disk was successfully reconstructed using reflected THz pulses and a digital processing algorithm. A number of assumptions were made in the use of the algorithm: first that the targets had no dispersion and diffraction, second that the reflection is so weak that multiple reflections could be ignored, and third that the refractive index is uniform in each layer examined. Such assumptions restrict the applicability of the tomographic technique and exclude the possibility of spectroscopic analysis.
Although computed tomography is well known in X-ray radiographic imaging, a serious problem in reconstructing an image using THz computed tomography is that the THz wave does not satisfy the short wave limit as the X-ray satisfies in X-ray computed tomography. For example, if one attempts to image a 10 cm target using a THz beam with a 10 cm Rayleigh range, the minimum THz beam size will have an electric field waist of 6 mm. Therefore, if one treats the beam as a ray line, the resolution will not be any better than 6 mm. This implies that the resulting image of the target only contains 30×30 pixels—an inadequate resolution. Therefore, a need remains for a THZ CT imaging system and imaging method that provide improved resolution compared with the resolution obtained using traditional X-ray computed tomography technology and that permit the use of the computed tomographic algorithm.